The fetus, placenta, and mother are interdependent and constitute a functional unit called the feto-placento-maternal unit or simply, the fetoplacental unit. The secretion of hormones by the placenta involves the shuttling of many of the hormonal substrates back- and-forth between the placenta, fetal circulation and maternal circulation. Following are the major examples of this shuttling.
(1) The placenta cannot synthesize cholesterol from acetate. However, both mother and fetus can do so and the cholesterol so formed diffuses into the placenta, which possesses the enzymes needed to convert cholesterol to progesterone via pregnenolone. The progesterone formed in the placenta diffuses back into the maternal circulation and exerts its physiological actions. Placental progesterone also diffuses into the fetus where it is converted to corticosteroids.
(2) The enzyme 17, 20 lyase is essential for the synthesis of DHEA (dehydroepiandrosterone), which is the precursor to all estrogens. 17, 20 lyase is absent in the placenta. Hence, for estrogen synthesis, the placenta obtains DHEAS from the maternal and fetal circulation. DHEAS (dehydroepiandrosterone sulfate) is deconjugated to DHEA in the placenta before it is converted to estriol.
(3) Estriol, the major circulating hormone in pregnancy is synthesized from DHEAS. The conversion requires mainly two enzymes: 16a hydroxylase and aromatase. However, 16a- hydroxylase is absent in the placenta. The placenta therefore takes up 16 OH-DHEAS from maternal and fetal circulation, deconjugates it into 16 OH-DHEA and converts it into estriol. Fetal 16 OH-DHEA is the major source of placental estriol and therefore, the urinary excretion of estriol in mother is an index of the health of the fetus.